Extra Charge

Hello.

A while back, I upgraded my phone. The representative set up my new phone for me. He thought I wanted a new number, so he originally gave me a new one for my new phone. But I just wanted to stay to my original number, so he reverted it back in less than a minute.

I checked my bill, and saw that I was being charged for 4 lines, instead of the 3 that I have. I spoke with a representative over the phone and it turns out he opened the promo line on the original number and never voided it when he switched back to my old number, which means I have to pay for a fourth line. Because of the extra line, I have to pay an extra $50 a month, and I've been doing this for months. I was told that they cannot fix this, because they cannot change the number of the installment, even if it was created in error, prompting a higher bill. I was told they could get rid of the extra bill if the phone is paid off, but I cannot afford an $800+ payment outright. 

Since I've been overcharged and cannot have my issue fixed, is there a way that my account gets some kind of refund, since this happened due to a representative's error? I do not want to pay $50 a month for the next 3 years for a phone number that does not exist in my lines.